1. Technical Field
The present invention relates to an oscillation circuit having a cross-coupled circuit and an oscillator having the oscillation circuit and a resonator.
2. Related Art
In the past, a Colpitts oscillation circuit was usually used as an oscillation circuit using a resonator. It is known that a gain of 4 or more is required for oscillation of the Colpitts oscillation circuit (for example, see “Design Application of Analog CMOS Integrated Circuit” (page 611), written by Seishiro MURATA and issued on Jul. 30, 2005). Since the gain is proportional to a negative resistance, the negative resistance proportional to the gain of 4 or more should be set to oscillate.
On the other hand, most of mobile communication devices and sensor networks are driven by a battery. Accordingly, it is preferable that the power consumption of an oscillation circuit is as low as possible while satisfying a desired operation. The current to be applied should be reduced to reduce the power consumption of the oscillation circuit. However, when the current is reduced, the gain is also reduced. Since the gain and the negative resistance are proportional to each other, the decrease in gain causes a decrease in negative resistance. Therefore, it is difficult to satisfy the oscillation condition.
A study for reducing inter-terminal capacitance of a resonator which is a reason for reducing the negative resistance, or minimizing a parasitic component in a circuit layout has been made to solve the above-mentioned problem, but did not increase the negative resistance.
In order to solve the above-mentioned problem, in JP-A-2006-60687, the negative resistance at a desired frequency is increased by providing a negative resistance increasing circuit. This circuit constitutes a feedback circuit for feeding back the output of an oscillation circuit through a capacitor, thereby increasing the negative resistance.
An oscillation circuit using a cross-coupled circuit is also known (for example, see U.S. Pat. No. 6,696,899). In U.S. Pat. No. 6,696,899, a resonator is connected between drains of a pair of active elements (MOS transistors) and the oscillation circuit is made to oscillate in an anti-resonance frequency of the resonator.
However, in JP-A-2006-60687, since a new circuit is required for increasing the negative resistance, the power corresponding to the power consumption of the new circuit is increased. Since the negative resistance is affected by the precision in feedback capacity, the negative resistance may get non-uniform, thereby not providing the desired negative resistance.
In general, it is known in an oscillation circuit using the anti-resonance frequency that the Q characteristic of the resonator is more deteriorated than that in the oscillation circuit using a resonance frequency. Accordingly, in the configuration described in U.S. Pat. No. 6,696,899, it is considered that it is difficult to embody an oscillation circuit having an excellent Q characteristic of a resonator.